A. Field of the Invention
Various embodiments of the invention relate to superhard surfaces and components of various compositions and shapes, methods for making those superhard surfaces and components, and products, which include those superhard surfaces and components. Such products include biomedical devices such as prosthetic joints and other devices. More specifically, some preferred embodiments of the invention relate to diamond and polycrystalline diamond bearing surfaces and prosthetic joints that include diamond and polycrystalline diamond bearing surfaces. Some preferred embodiments of the invention utilize a polycrystalline diamond compact (xe2x80x9cPDCxe2x80x9d) to provide a very strong, low friction, long-wearing and biocompatible bearing surface in a prosthetic joint. Any bearing surface, including bearing surfaces outside the field of prosthetic joints, which experience wear and require strength and durability will benefit from embodiments of the invention.
B. Description of Related Art
This section will discuss art related to prosthetic joint bearing surfaces. Artificial joint replacement has become a widely accepted successful medical practice in the treatment of arthritic or deformed joints. Hundreds of thousands of joint replacement procedures are performed every year. Prosthetic hip and knee replacement comprise the vast majority of these procedures, however many other joints are also treated as well including, but not limited to, the shoulder, elbow, wrist, ankle, and temparomandibular joints. Additionally, there are other joints, such as the intervertebral disk joint of the spine, which are not commonly replaced with prosthetic joints, but which might be amenable to such treatment to remedy disease states if sufficiently durable materials in functional designs were available.
The ideal total artificial joint prosthesis can be characterized in terms of flexibility, durability, and compatibility. Flexibility: An ideal total joint prosthesis should restore a normal range of motion, allowing all activities possible with a normal natural joint without an increase in the relative risk of dislocation. Durability: The mechanical parts of the articulation should function without wearing out or breaking, and the implant""s fixation to the recipient""s skeleton should remain rigidly intact for the duration of the recipient""s lifetime, without requiring restrictions on the intensity of activities or the degree of load bearing beyond those applying to a natural joint. Compatibility: The prosthetic materials and wear byproducts should be biocompatible, and should not elicit toxic, inflammatory, immunologic, or other deleterious reactions in the host recipient. Currently available devices fall short of fulfilling these criteria in one or more significant ways. It is the objective of the current invention to improve upon the prior technologies in terms of meeting these criteria.
In general, there are two types of artificial jointsxe2x80x94articulating joints and flexible hinge joints. Articulating joints include hip, knee, shoulder, ankle and other joints. Flexible hinge joints include silastic and metacarpal-phalangeal joints. In the past, articulating joints have consisted typically of a hard surface (metal or ceramic) mated to a plastic surface (ultra high molecular weight polyethylene). Other joints have been composed of variations of hard on hard articulations (metal on metal and ceramic on ceramic). Articulating joints may take a myriad of configurations including variations on a ball in socket design, such as with a hip and shoulder joint, and variations on a hinge design as with a knee, elbow, or metacarpal-phalangeal joint. In every case, the prosthesis is designed to restore to the greatest extent possible, the functional range of motion, and mechanical stability of the affected joint.
As a detailed example of problems found in the prior art, we will review the hip joint. It includes a convex spherical ball (femoral head) and a concave socket (acetabular socket) articulation. Hip joint replacement consists of replacing the damaged articular surfaces with new articulating bearing surfaces. On the acetabluar side, a hemisphere-like cup is placed in the patient""s damaged or worn socket, and fixed by some means to the patient""s bone. On the femoral side, the prosthetic replacement consists of a sphere-like ball designed to fit into, and articulate with the prosthetic acetabular cup. The sphere-like ball may be a resurfacing device designed to fit over the patient""s own femoral head (so called xe2x80x9csurface replacementxe2x80x9d). Or more commonly it consists of a ball attached to a stem, which is inserted into the femoral canal anchoring the prosthesis to the patient""s femur. The ball and socket work as a pair in similar fashion to the original hip, restoring a partial range of linear and rotational motion.
Alternatively, only a surface replacement or a ball and stem set are provided without a corresponding socket for a hemiarthroplasty procedure (discussed below). For total hip joint replacement, the most commonly used device consists of a metal head articulation with a high density ultra high molecular weight polyethylene (UHMWPE) surface, but ceramic (alumina, and partially stabilized zirconia) heads are also used, having certain advantages as well as disadvantages relative to their metal counterparts. Metal on metal, and ceramic on ceramic articulations are also used in routine medical practice elsewhere in the world, and are being used on an investigational basis in the United States.
Replacement of only one half of the hip joint is called hemiarthroplasty. This is performed when only one of the articulating portions of the joint is damaged, as with avascular necrosis of the femoral head, or in the case of a hip fracture that is not amenable to repair. The damaged portion is replaced with a prosthetic articulation designed to function with the remaining natural biological portion of the joint. The requirements are somewhat different here than with a total articular replacement, in that the artificial portion must match the contours of the anatomic segment, and must be conducive to preservation of the function of the natural segment. This would include having a surface smooth enough to minimize wear and tear to the natural joint surface, and optimization of surface material properties and contours that would encourage entrainment of joint fluid into the joint space. This entrainment of synovial fluid is essential to minimize wear to, and maintain nutrition and function of the biological joint surface.
Prosthetic joint implants must be securely anchored to the recipient""s bone to function properly. This fixation may be achieved through the use of cementing agents, typically consisting of polymethylmethacrylate cement, through biological fixation techniques including direct osseointegration to metal or ceramic fixation surfaces and bone ingrowth into porous surfaces on implant surfaces, or through a mechanical interference press fit between the implant and the host bone. Preservation and maintenance of this secure fixation is critical to the long-term success of the prosthetic construct.
When evaluating prior art technology relative to the criteria previously established for an ideal prosthetic joint, we find that metal balls articulating with polyethylene cups do not provide optimal results. Due to geometric restrictions on the implant design imposed by implant material properties, and anatomic constraints, artificial hips have a decreased safe range of motion compared to normal natural counterparts. The polyethylene bearings may wear through after between 5 and 20 years of service, depending upon factors such as patient age, weight and activity level. The particulate debris resulting from this normal wear often results in inflammatory reactions in the bone surrounding and anchoring the implants, resulting in severe erosion of the bone. This is called xe2x80x9costeolysisxe2x80x9d and has proven to be a most prevalent cause of failure and subsequent artificial joint replacement.
The normal metal to ultra high molecular weight polyethylene (xe2x80x9cUHMWPExe2x80x9d) articulation of artificial joints results in the generation of billions of submicron polyethylene wear particles. It is the accumulation of this wear-related debris and the immune system""s reaction to it that results in the inflammatory response, which causes osteolysys. It is also the cumulative effect of this continual wear of UHMWPE that results in wear through of the mechanical joint and bearing failure. The younger and more active the patient, the shorter the anticipated functional life of the implant. Thus, those patients who, because of their youth, need the greatest durability from their implants, typically have the least durability.
Osteolysis can cause loosening of the critical implant-bone fixation, and may result in increased risk of fracture of the bone around the implants. Wear through of the components and/or periprosthetic osteolysis of the host bone with associated implant loosening and/or periprosthetic bone fracture requires major surgical intervention to remove the failed implants, reconstruct the damaged bone, and replace the failed prosthesis with a new artificial joint. This revision surgery is typically much more complicated than the initial implant surgery, and carries with it increased risks for perioperative complications, as well as increased risks for implant failure as compared to primary artificial joint replacement. Subsequent failures require further complex surgical intervention, with continually increasing risks of perioperative complications and early implant failure with each episode.
In order to reduce the risks of dislocation, recipients of artificial hips must restrict their range of motion in normal activities, compromising their ability to engage in many routine activities possible with normal natural joints. In order to decrease the rate of bearing wear which leads to implant failure due to bearing wear through and/or problems resulting from debris-related osteolysis, they must also restrict their activities in terms of intensity, and duration relative to that routinely possible with normal natural joints.
In an effort to reduce the risk of dislocation, larger diameter bearings have been tried where the recipient""s anatomy permits use of larger components. Surface replacement lies at the limit of this approach, and employs large bearings covering the patient""s own femoral head remnant, articulating with a relatively thin UHMWPE acetabular component. Use of larger diameter bearings results in some increase in safe range of motion of the joint. Unfortunately, in the metal/UHMWPE bearing couple, increasing bearing diameter leads to increased rates of debris generation together with increased risk of its associated problems. In the case of surface replacements, the thin UHMWPE is particularly susceptible to accelerated wear, osteolysis, and failure.
The prior art includes many proposed improvements over the typical metal ball and polyethylene cup articulation seeking to decrease these problems of limited motion, wear, and debris-related osteolysis.
Ceramic bearings have some advantages over prior art metals in a prosthetic joint system. Ceramic bearings have an increased wettability compared to metal, resulting in better boundary layer lubrication, and they are resistant to the wear-promoting scratches that can develop in metal heads in the course of normal wear and tear in the joint. Both of these factors have contributed to the lower rates of wear and debris generation observed with ceramic on UHMWPE seen in both laboratory and clinical studies.
Unfortunately, ceramic bearings are structurally brittle. This limits the number of sizes and neck lengths that can be safely employed in reconstruction, restricting the options available to the surgeon to complete an optimal mechanical reconstruction during surgery. This intrinsic material brittleness can also lead to sudden implant fracture under impact, resulting in sudden and often catastrophic implant failure. Ceramic bearings also suffer from geometric design constraints similar to their metal-polyethylene counterparts, and have a similar susceptibility to dislocation if restrictions on range of motion are violated by the recipient. The limitations in ceramic material properties do not permit the fabrication of surface replacement bearings.
More recently, attention has turned to UHMWPE in an effort to improve the longevity of these bearing couples. Most early efforts to alter fabrication techniques, such as hot pressed components in hip and knee systems, and efforts to modify material structure, such as the addition of carbon fibers and the use of a hipping process to increase crystalinity, have resulted in no demonstrable improvements in clinical or in vitro performance, and in fact, have often resulted in poorer wear characteristics. Other techniques have improved function to a limited measurable extent, such as injection molding of components.
It has been found that the most common sterilization technique used to prepare UHMWPE components for implantation has had extreme unanticipated effects upon the material properties and wear characteristics of this material, resulting in accelerated wear and early failure in many cases. Study of this phenomenon, which includes the generation of chemical cross-links in polyethylene chains, and the generation of persistent free radicals within the polymer has led to further inventions to eliminate the deleterious effects of this process, while possibly taking advantage of potential beneficial effects that may actually improve the wear characteristics of polyethylene. These most recent developments, while demonstrating promising results in laboratory simulation studies, have yet to demonstrate improved function in widespread, long-term clinical studies. If these new polyethylene technologies do result in demonstrable improvements in function, the intrinsic problems of metal and ceramic counter bearings may still adversely affect long-term durability. Ultimate strength of UHMWPE (organic polyethylene bonds) in tension, compression and shear are low in comparison with metals, ceramics and diamond bonds. Diamond resistance to wear exceeds that of all other materials. The table below compares properties of polycrystalline diamond compact with some other materials from which bearing surfaces could be made.
In order to avoid the potential problems of polyethylene entirely, others have turned to ceramic on ceramic and metal on metal contact surfaces. Ceramic on ceramic articulations have demonstrated improved wear rates, and excellent biocompatibility. However they suffer from the intrinsic limitation in material properties seen with ceramic heads used with polyethylenexe2x80x94brittleness and fracture risk. In addition, there is a tendency to develop catastrophic accelerated wear when a third body wear particle of sufficient hardness (such as another fragment of ceramic) is introduced into the articulation. Finally, the material property limitations of ceramic impose minimum material dimensional thicknesses that preclude the use of larger bearings or application as a surface replacement that would result in gains in effective range of motion.
Metal on metal bearings have also demonstrated improved volumetric rates of wear. And their material properties do make them suitable for application in large bearing applications and surface replacements effectively addressing the need for increased safe range of motion, and decreased risk of dislocation. However, concern still exists over the character of the wear debris of this metal-metal bearing couple. Though volumetric wear is quite low compared to polyethylene, particle size is extremely minute, on the order of 40-100 angstroms, resulting in an even larger total number of particles that with UHMWPE. These wear particles consist of cobalt-chrome-molybdenum alloy, which, with their extraordinarily large combined surface area, can result in significant release of metals ions with documented toxicity, and potential for long term carcinogenicity. It remains for long-term clinical studies to document the actual risk of this exposure, but significant questions have been raised with regard to this issue. As with ceramic on ceramic articulations, metal-to-metal bearings are susceptible to accelerated wear from third body wear particles.
Thus, the failures and pervasive defects of the prior art show a clear need for improved prosthetic joints. The various embodiments of the invention address the many deficiencies left by the prior art by providing prosthetic joints which are very long lasting, strong, have a low coefficient of friction, are biocompatible, experience little or no wear, and do not shed significant amounts of particles during use.
It is an object of some embodiments of the invention to provide components for prosthetic joint implants having increased wear resistance and a decreased coefficient of friction, therefore having maximum life of the replaced joint. It is a feature of some embodiments of the invention that diamond of various types and other superhard materials are used for the bearing surfaces of the joint, the superhard materials including diamond being very resistant to wear and having a very low coefficient of friction. For the purposes of this document, a superhard material is a material that has a Knoop hardness of at least about 4000. This includes diamond (whether natural diamond or synthetic diamond), cubic boron nitride and wurzitic boron nitride. It is a consequent advantage that the joint will likely not wear out during the lifetime of the user and will only generate insignificant amounts of benign wear particles.
It is an object of some embodiments of the invention to provide a prosthetic joint that does not shed significant amounts of debris or wear particles as a result of use or wear. It is a feature of some embodiments of the invention that polycrystalline diamond compacts or other superhard materials form at least one of the articulation surfaces of the joint, resulting in a low friction and long wearing joint that sheds little to no debris or particles during use. Therefore a lessened risk of osteolysis is a significant advantage of these embodiments of the invention.
It is another object of some preferred embodiments the invention to use the hardest materials known to man, namely diamond, cubic boron nitride and other superhard materials to give prosthetic joints the highest resistance to wear currently known to man. It is a feature of the invention that some preferred embodiments use polycrystalline diamond compact (xe2x80x9cPDCxe2x80x9d) for a bearing surface. For the purposes of this document, a polycrystalline diamond compact includes a volume of synthetic diamond attached to a substrate material. The polycrystalline diamond is extremely hard and, when polished, has one of the lowest coefficients of friction of any known material. It is a consequent advantage of the invention that the joint life far exceeds that of the recipient. The polycrystalline diamond compact may be manufactured by a variety of methods, including high pressure and high temperature sintering in a press, chemical vapor deposition, physical vapor deposition, and others.
It is another object of some embodiments of the invention to provide joint components, which are completely biocompatible. The most preferred material used in some embodiments of the invention, polycrystalline diamond compact, is extremely biocompatible and elicits minimal or negligible immune response or other attack by the body. It is an advantage that the joint will be substantially less disruptive to the body""s systems than prior art joints.
It is another object of some embodiments of the invention to provide improved geometry within the joint space in order to allow optimal utilization of superhard materials, including polycrystalline diamond compacts. The improved geometry is intended to limit stresses both residual and those imposed during service use. Due to the very hard and strong nature of the materials being utilized, the entire joint geometry can be optimized for a particular prosthetic application, rather than simply copying existing prosthetic joints with new materials.
It is another object of some embodiments of the invention to provide an artificial joint with an improved ball and cup configuration. The invented prosthetic hip joint is made from very few components in a simple design, thus contributing to the ease of manufacturability and reliability. The ball and cup are designed to maximize articulation within the normal range of movement for a human joint while providing exceptional wear resistance and useful life.
It is another object of some embodiments of the invention to provide a cup and ball type prosthetic joint that can be used without a separate shell. Some embodiments of the invention provide an acetabular cup that may be fixed to a patient""s bone without the need for a separate shell.
It is another object of some embodiments of the invention to provide a prosthetic joint, which combines the use of a polycrystalline diamond compact bearing surface with a counter-bearing surface of another material. In some embodiments of the invention, the counter-bearing surface may be any of a wide variety of materials, including prior art UHMWPE, or even the patient""s natural cartilage in the case of a hemiarthroplasty procedure.
It is another object of some embodiments of the invention to provide a modular prosthetic joint assembly with a superhard bearing surface. Some embodiments of the invention provide modular prosthetic joint components which can be selected and assembled at the time of surgery to provide a prosthetic joint system with dimensions and angular orientation closely approximating those of the patient""s natural joint. An appropriate superhard bearing surface may be provided, such as a polycrystalline diamond compact.
It is another object of some embodiments of the invention to provide a prosthetic bearing surface useful in hemiarthroplasty procedures. The nature of the preferred prosthetic joint articulation surface bearing material, polycrystalline diamond, is such that it can be used as a bearing material that can articulate against natural cartilage, permitting hemiarthroplasty.
It is another object of some embodiments of the invention to provide a replacement liner or cover for a natural joint. In some embodiments of the invention, a femoral head or other natural joint surface may be resurfaced or relined with the invention in order to achieve bone conservation.
It is another object of some embodiments of the invention to provide polycrystalline diamond compacts useful as bearing surfaces with improved fastening strength between the diamond table and the substrate of the polycrystalline diamond compact. In various embodiments of the invention, topographical features are provided on the substrate in order to achieve this improved fastening strength.
It is an object of some embodiments of the invention to provide substrate configuration that permit the manufacture of spherical, partially spherical and arcuate polycrystalline diamond compacts. Various substrate configurations are disclosed which achieve this object.
It is an object of some embodiments of the invention to provide non-planar polycrystalline diamond compact bearing surfaces. Various embodiments of the invention provide novel bearing surfaces that are non-planar and preferably are manufactured as polycrystalline diamond compacts.
It is an object of some embodiments of the invention to provide a method for manufacturing non-planar polycrystalline diamond compact bearing surfaces. Various methods are disclosed for materials preparation and polycrystalline diamond compact manufacturing that will produce non-planar polycrystalline diamond compact bearing surfaces, including but not limited to concave and convex spherical bearing surfaces.
It is an object of some embodiments of the invention to provide methods for rough shaping of non-planar polycrystalline diamond compact bearing surfaces. Novel machining techniques are disclosed which accomplish such shaping.
It is an object of some embodiments of the invention to provide methods for finish polishing of non-planar polycrystalline diamond compact bearing surfaces. Novel polishing techniques are disclosed which permit polishing of polycrystalline diamond compact bearing surface to be highly polished to a low coefficient of friction.
It is another object of some embodiments of the invention to provide a joint with enhanced wettability. Use of a diamond bearing surface achieves this object.
The objects, features and advantages of the inventions mentioned above are exemplary and illustrative only so that the reader may begin to perceive advantages to be accrued by use of the invention alone or in combination with other technology. Additional objects, features and advantages of the invention will become apparent to persons of ordinary skill in the art upon reading the specification and claims and viewing the drawings.